Handle projects at GitHub whose main branch is not 'master'.

It would be useful to be able to specify branch(es) to pay attention towhen adding a new project, since not all projects use `master` as their main development branch.

For example, NYCComptroller/Checkbook does their main development on the `checkbook_nyc` branch right now.  But in http://opengovhacknight.org/projects.html, the project looks like it has very low commit activity, because only `master` is examined.

The commit here is an untested draft -- I don't expect that it works out-of-the-box, and didn't have a GITHUB_TOKEN set up to test with (sorry).  But sometimes a patch is the best way to express an idea, so here it is!  I hope it's a useful start.